Pluto has more in common with the inner planets than with its gas giant neighbors. Pluto is a small, rocky planet with very little atmosphere.
Little is known about Pluto. It is thought to have a very large, rocky core. The core could have a radius over 575 miles (925 km) or about 79 percent of Pluto's radius. Pluto's mantle is probably a frozen ocean of water, nitrogen, methane, and other ices. This mantle of frozen gases could be about 145 miles (235 km) thick or about 20 percent of Pluto's radius. If Pluto has a crust, it is probably only about one percent of the planet's total radius. Pluto's atmosphere may just be methane, carbon monoxide, and nitrogen evaporating from the surface and escaping into space.
Surface Features
Not much is known about the surface of Pluto. It is thought to be completely covered in a snow of frozen methane and other gases. Recent photos, however, show darker areas near the equator. This suggests that Pluto may have areas where rock and other material are at the surface. Pluto's surface appears slightly reddish or orange. The color may be caused by the mixture of ices that make up the surface.
Atmosphere
Pluto may have an atmosphere only at certain times of the year. Pluto's orbit is very eccentric. So, Pluto's distance from the Sun varies throughout Pluto's year. Pluto is sometimes nearer the Sun than at other times in its orbit. When Pluto is nearer the Sun, some frozen nitrogen, methane, and carbon monoxide from the surface may change to a gas. This evaporation creates a temporary atmosphere. As Pluto's orbit moves it away from the Sun, these gases may freeze and fall like snow back to the surface. When Pluto is farthest from the Sun, the planet may have almost no atmosphere.
Pluto does not have a thick atmosphere to absorb and scatter the Sun's light. So, the Sun's light reflects off Pluto's icy surface. Pluto's albedo is .050. Pluto's slight atmosphere doesn't scatter or block any of the little sunlight Pluto receives. So, the sky is always dark.
Orbit and Rotation
Orbit
It takes Pluto over 248 years to orbit the Sun. One reason Pluto takes so long to orbit the Sun is the length of its orbit. Pluto's orbit covers a distance of 23 billion miles (37 billion km). That's about twice as far as Uranus and four times farther than Saturn. Another reason is the farther a planet is from the Sun, the slower it moves. Pluto is the slowest-moving planet. Pluto's orbital velocity is only 10,500 mph (16,900 kph). Like all the planets, Pluto's orbit is elliptical. Pluto's orbit is the most eccentric of all the planets. Pluto's average distance from the Sun is 3.7 billion miles (6 billion km). At its farthest, Pluto is 4.6 billion miles (7.4 billion km) from the Sun. At its closest, Pluto is 2.76 billion miles (4.44 billion km) from the Sun. At that point, Pluto is closer to the Sun than Neptune is.
Pluto's elliptical orbit keeps it inside Neptune's orbit for 20 years of its 248-year orbit. Pluto last crossed inside Neptune's orbit in 1979. Pluto won't move outside Neptune's orbit until 1999. Pluto and Neptune will never collide because their orbital paths are not in the same plane. At the two points where their paths are in the same plane, their orbital paths are thousands of miles apart.
Pluto's orbital plane is inclined 17.2¬° to the ecliptic. That's the largest inclination of any planet.
Rotation
Pluto takes 6 days and 9 hours to rotate once on its axis. Unlike most of the planets, Pluto has a retrograde rotation.
Pluto's axial tilt is 57.5¬°. Only Uranus has more of an axial tilt. Although Earth's axial tilt causes the seasons, Pluto is so far from the Sun that its tilt has no effect.
Moons and Rings
Pluto has no rings, but it does have a moon. The moon, called Charon, orbits Pluto at a distance of only about 11,400 miles (18,300 km). It orbits Pluto every 6 days and 9 hours.
The diameter of Charon is about 750 miles (1,210 km). That makes it a little more than half the size of Pluto. Most moons are no more than one-fourth the size of their planets. Since Charon is so close to Pluto and so large, it appears huge in Pluto's sky. From Pluto, Charon would look about eight times larger than the full moon looks from Earth. Charon takes the same time to rotate, or spin on its axis, as it does to orbit Pluto. So, like our Moon, only one side of Charon ever faces Pluto. The length of Pluto's day is the same as Charon's. That means one side of Pluto is always facing Charon. If you were on either Charon or Pluto, the other would seem never to move in the sky. Charon is composed mostly of water-ice. Although both Charon and Pluto are covered with ice, Charon's gray surface is less reflective and doesn't have any color. That's because Charon's surface is made of dirty water-ice. Pluto is 50 percent brighter than Charon.
Every 124 years (twice in Pluto's revolution around the Sun), Charon's orbit lines up directly in front of Pluto. This results in a cycle of eclipses that lasts several years. As Charon circles behind Pluto and back in front every six days, hundreds of eclipses occur. The last eclipse cycle occurred from 1985 to 1990. In the year 2109, the next cycle will begin.
Magnetic Field
It isn't known if Pluto has a magnetic field. A magnetic field is produced when electrically conductive materials move. The movement creates an electrical current. To generate electricity, Pluto would have to have more metal in its core than it is believed to have. Also, Pluto would have to be at least partly liquid instead of solid. So, it's unlikely that Pluto has a magnetic field.
Gravity
Pluto is a tiny planet. It also doesn't have much mass. Earth's mass is about 450 times greater than Pluto's. Pluto's surface gravity is only 4 percent of Earth's surface gravity. Pluto has a weaker gravitational pull than any other planet in the solar system.
